.\"	$OpenBSD: BIO_read.3,v 1.8 2018/03/27 17:35:50 schwarze Exp $
.\"	OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400
.\"
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.Dd $Mdocdate: March 27 2018 $
.Dt BIO_READ 3
.Os
.Sh NAME
.Nm BIO_read ,
.Nm BIO_gets ,
.Nm BIO_write ,
.Nm BIO_puts
.Nd BIO I/O functions
.Sh SYNOPSIS
.In openssl/bio.h
.Ft int
.Fo BIO_read
.Fa "BIO *b"
.Fa "void *buf"
.Fa "int len"
.Fc
.Ft int
.Fo BIO_gets
.Fa "BIO *b"
.Fa "char *buf"
.Fa "int size"
.Fc
.Ft int
.Fo BIO_write
.Fa "BIO *b"
.Fa "const void *buf"
.Fa "int len"
.Fc
.Ft int
.Fo BIO_puts
.Fa "BIO *b"
.Fa "const char *buf"
.Fc
.Sh DESCRIPTION
.Fn BIO_read
attempts to read
.Fa len
bytes from BIO
.Fa b
and places the data in
.Fa buf .
.Pp
.Fn BIO_gets
performs the BIOs "gets" operation and places the data in
.Fa buf .
Usually this operation will attempt to read a line of data
from the BIO of maximum length
.Fa len No - 1 .
There are exceptions to this however, for example
.Fn BIO_gets
on a digest BIO will calculate and return the digest
and other BIOs may not support
.Fn BIO_gets
at all.
The returned string is always NUL-terminated.
.Pp
.Fn BIO_write
attempts to write
.Fa len
bytes from
.Fa buf
to BIO
.Fa b .
.Pp
.Fn BIO_puts
attempts to write a null terminated string
.Fa buf
to BIO
.Fa b .
.Pp
One technique sometimes used with blocking sockets
is to use a system call (such as
.Xr select 2 ,
.Xr poll 2
or equivalent) to determine when data is available and then call
.Xr read 2
to read the data.
The equivalent with BIOs (that is call
.Xr select 2
on the underlying I/O structure and then call
.Fn BIO_read
to read the data) should
.Em not
be used because a single call to
.Fn BIO_read
can cause several reads (and writes in the case of SSL BIOs)
on the underlying I/O structure and may block as a result.
Instead
.Xr select 2
(or equivalent) should be combined with non-blocking I/O
so successive reads will request a retry instead of blocking.
.Pp
See
.Xr BIO_should_retry 3
for details of how to determine the cause of a retry and other I/O issues.
.Pp
If the
.Fn BIO_gets
function is not supported by a BIO then it is possible to
work around this by adding a buffering BIO
.Xr BIO_f_buffer 3
to the chain.
.Sh RETURN VALUES
All these functions return either the amount of data successfully
read or written (if the return value is positive) or that no data
was successfully read or written if the result is 0 or -1.
If the return value is -2, then the operation is not implemented
in the specific BIO type.
The trailing NUL is not included in the length returned by
.Fn BIO_gets .
.Pp
A 0 or -1 return is not necessarily an indication of an error.
In particular when the source/sink is non-blocking or of a certain type
it may merely be an indication that no data is currently available and that
the application should retry the operation later.
.Sh SEE ALSO
.Xr BIO_meth_new 3 ,
.Xr BIO_new 3 ,
.Xr BIO_should_retry 3
.Sh HISTORY
.Fn BIO_read ,
.Fn BIO_gets ,
.Fn BIO_write ,
and
.Fn BIO_puts
first appeared in SSLeay 0.6.0 and have been available since
.Ox 2.4 .
